Switch format = "qs" to the qs2 package

[wlandau]

c.f. qsbase/qs#103.

We can do this back-compatibly. New targets will be saved with qs2::qs_save(). For existing targets, we can first try qs2::qs_read() and then fall back on qs::read(). qs2::qs_read() automatically detects the legacy qs format and throws an informative error.